1. Technical Field of the Invention
The present invention relates to a high-frequency power device for supplying power to a load such as a plasma processing unit, etc., which is employed for applications accompanying discharge such as, for example, plasma etching, plasma CVD, etc.
2. Description of the Related Art
As a high-frequency power device for supplying power to a load accompanying discharge, a high-frequency power device is available, which varies the oscillation frequency of an internal oscillation circuit 52 before and after commencement of discharge in a load.
FIG. 19 is a view depicting a configuration of a conventional high-frequency power device 50 and a connection between a high-frequency power device 50, an automatic matching unit 6, and a load 5. (see, for example, Japanese Published Unexamined Patent Application No. Hei-9-161994)
The high-frequency power device 50 outputs high-frequency power using a power amplifier 53, and the output value of high-frequency power detected by a power measuring unit 54 is controlled so that the output value becomes a target value established by a power setting unit 56. Also, the output frequency of the high-frequency power device is determined by the oscillation frequency of the oscillation circuit 52 whose oscillation frequency is controlled by a frequency controlling circuit 51. The high-frequency power output from the high-frequency power device 50 is supplied to the load 5 via a transmission line 2 composed of a coaxial cable, an automatic matching unit 6 and a load connection portion composed of a shielded copper plate. In addition, an ON/OFF controlling circuit 58 controls to turn on and off the output of the oscillation circuit 52. The oscillation circuit 52 outputs a high-frequency signal when an ON signal is output from the ON/OFF controlling circuit 58. The ON/OFF controlling circuit 58 is controlled by operating a power output switch (not illustrated) provided in the high-frequency power device 50 or is controlled by a control signal from an external device.
The automatic matching unit 6 is employed for the purpose of efficiently supplying high-frequency power to a load by matching both of impedance, which are the power side impedance Zo (usually 50Ω) which is observed from the input terminal of the automatic matching unit 6 to the high-frequency power device 50 side via the transmission line 2, and the load side impedance ZL (impedance of the automatic matching unit 6, the load connection portion 4 and the load 5) which is observed from the input terminal of the automatic matching unit 6 to the load side. The automatic matching unit 6 is internally provided with a variable impedance element (for example, a variable capacitor, variable inductor, etc.,) (not illustrated), wherein the automatic matching unit 6 has a feature of varying impedance of the variable impedance element so that the power device side impedance Zo and the load side impedance ZL are impedance-matched to each other.
The load 5 is an apparatus that is provided with a processing section and processes (etches, CVD-deposits, etc.) a workpiece such as a wafer, liquid crystal substrate, etc., brought into the interior of the processing section. In order to process the workpieces, a plasma discharge gas is introduced into the processing section, and high-frequency power (voltage) supplied from the high-frequency power device 50 is applied to the plasma discharge gas, the load 5 discharges the plasma discharge gas to bring a non-plasma state into a plasma state. And, the workpieces are processed by utilizing the plasma. Further, a load accompanying such a discharge is called a “discharge load.”
The discharge detecting unit 57 detects existence of discharge in a load and outputs a detection signal to the frequency controlling circuit 51 and the automatic matching unit 6. Since the frequency controlling circuit 51 and the automatic matching unit 6 are able to judge, based on the detection signal, whether or not discharge occurs in the load, it is possible to vary the control, depending on the state of discharge. Next, a description is given of the control.
In the discharge load described above, it has been known that the load side impedance ZL which is observed from the high-frequency power device 50 to the load side radically changes before and after commencement of discharge in the load. However, the variable impedance element of the automatic matching unit 6 described above cannot significantly vary the impedance instantaneously in view of its structure. In order to significantly vary the impedance by using the variable impedance element of the automatic matching unit 6, time of one second through several seconds is required in some cases.
Therefore, since the automatic matching unit 6 cannot follow a radical change in impedance if the impedance is matched by the automatic matching unit 6 during the time, there may be cases where no matched state is brought about immediately after commencement of discharge. As a result, the reflection is increased, power necessary to maintain discharge cannot be supplied to the load, wherein there may be a problem that plasma generated by discharge, etc., disappears.
To the contrary, since the output frequency of the high-frequency power device 50 can be instantaneously varied, it is possible to instantaneously match the impedance by varying the load side impedance ZL by changing the output frequency. Therefore, in the conventional high-frequency power device 50, such a control has been employed, which varies the load side impedance ZL by varying the output frequency of the high-frequency power device 6 without using the automatic matching unit 50 when the impedance radically changes before and after commencement of discharge. That is, such a control as shown in Steps 1 through Step 3 below is carried out. Also, it is sufficient to vary the oscillation frequency of the oscillation circuit 52 in order to vary the output frequency of the high-frequency power device 50. In the description of the following Step 1 through Step 3, the oscillation frequency of the oscillation circuit 52 is used for description regarding a change in the output frequency of the high-frequency power device, in order to describe a detailed control method.
Step 1: Until commencement of discharge, the oscillation frequency of the oscillation circuit 52 is varied so as to be impedance-matched to the resonance frequency of the load side impedance ZL before commencement of discharge. That is, discharge is commenced with the load side impedance ZL which is easy to commence the discharge. Also, at this time, the variable impedance element of the automatic matching unit 6 does not operate.
Step 2: After commencement of the discharge, the oscillation frequency of the oscillation circuit 52 is set to a predetermined fixed frequency suitable for discharge. The oscillation frequency is obtained by simulations and experiments, etc.
Step 3: High frequency is output at the oscillation frequency established in Step 2, and the impedance is matched by using the automatic matching unit 6.
In the above-described high-frequency power device 50 capable of varying the output frequency, it is possible to cope with a radical change in the load side impedance ZL by varying the output frequency even before and after commencement of discharge. However, since respective components such as a variable impedance element in the load and automatic matching unit have different characteristics in connection with the frequency, the load side impedance differs, depending on the states of the respective components at that time, even with the same change in the output frequency. For example, where the high-frequency power device 50 in which the variable range of the output frequency is 13.56±0.5 MHz is used, and the load side impedance ZL is 30+j30Ω, when the output frequency is increased by 0.04 MHz from 13.56 MHz to 13.60 MHz, the load side impedance ZL becomes 50+j0Ω in the state A, but it becomes 20+j20Ω in the state B.
That is, since there is no relationship between the output frequency of the high-frequency power device 50 and the load side impedance ZL, it is not possible that the load side impedance ZL is measured, and the output frequency of the high-frequency power device 50 is varied in association with the measured value. For example, when the load side impedance ZL becomes 30+j30Ω, it is not always possible to match the impedance by setting the frequency to 13.60 MHz, wherein no association can be established. Accordingly, in order to match the impedance by varying the output frequency, it is necessary to vary the output frequency at random.
In the conventional high-frequency power device 50, due to the above-described situations in terms of control, it is considered that such controls as described in Steps 1 through 3 are carried out. That is, in Step 1, since the discharge is before commencement, almost the entire reflection state is brought about. Therefore, even if the output frequency is varied at random, no adverse influence is brought about until the discharge is commenced with the impedance matched. Rather, since the output frequency can be instantaneously varied, it is possible to match the impedance in a shorter time than matching the impedance by the automatic matching unit employing a variable impedance element. In step 2, since the load side impedance ZL radically changes immediately after the discharge is commenced, it is further preferable to match the impedance by frequency conversion because the impedance can be instantaneously matched. In Step 3, since the impedance is matched after the discharge is commenced, it is difficult to match the impedance by changing the output frequency due to the above-described situations. Therefore, the impedance is matched by using the automatic matching unit 6.
Thus, in regard to a problem that the impedance radically changes before and after commencement of discharge in the conventional high-frequency power device 50, the problem can be solved by varying the output frequency of the high-frequency power device. However, in connection to matching of the impedance after commencement of discharge, the automatic matching unit 6 is still used, wherein there are the following problems.
(1) Since the variable impedance element used in the automatic matching unit 6 is provided with a movable portion, the movable portion is subjected to wearing, wherein mechanical longevity is reached. For example, where a variable capacitor is used as the variable impedance element, since the axis of the variable capacitor connected to a motor rotates, the sliding portion between the axis of the variable capacity and a bearing is subjected to wearing, and the wearing reaches mechanical longevity. In particular, since, in the discharge load, the impedance of the load always fluctuates, the variable impedance element also always moves. Accordingly, the wearing degree is made large. Also, since the movement of the variable impedance element differs under the conditions of the load, etc., the period of time to mechanical longevity differs in the respective variable impedance elements. In this connection, as the variable impedance element reaches mechanical longevity, it is necessary that the variable impedance element is replaced or the automatic matching unit 6 is replaced. In this case, it is necessary to carry out replacement work once the production line is stopped. Based on such reasons, it is desired that replacement of the variable impedance element and the automatic matching unit 6 is eliminated. However, in the automatic matching unit 6, since the variable impedance element is employed, there may be cases where the replacement is unavoidable when the components thereof reach mechanical longevity.
(2) In recent years, a demand for lower production costs in the entire apparatus including the high-frequency power device 50 and a demand for downsizing thereof have increased. Since, in the automatic matching unit 6, the variable impedance element is expensive and a motor is required to drive the variable impedance element, the automatic matching unit 6 becomes remarkably expensive, resulting in an increase in the production costs of the entire apparatus including the high-frequency power device 50. In addition, since the structure of the automatic matching unit 6 is complicated and the variable impedance element which is a major part thereof is large-sized, it is difficult to downsize the apparatus.
(3) A demand for further higher speed has been brought about in impedance matching during discharge after the discharge is commenced. However, since the automatic matching unit 6 is conventionally used, the internal variable impedance element cannot instantaneously operate, wherein there may be cases where time of one second through several seconds is required until the impedance is matched.
(4) Further, in the conventional high-frequency power device 50, discharge in the load 5 is detected by using the discharge detecting unit 57. There are various systems of the discharge detecting unit 57, for example, a system for detecting light emission when discharge occurs, a system for detecting a direct current voltage generated in load, etc., are available. However, if the discharge detecting unit 57 is used, there is a problem that the configuration of the high-frequency power device 50 is made complicated. For example, in the system for detecting light emission when discharge occurs, since it is necessary to attach the discharge detecting unit 57 in the vicinity of the load 5, and it is necessary to provide a signal line between the discharge detecting unit 57 and the main body of the high-frequency power device 50, the configuration becomes complicated. In addition, the discharge detecting unit 57 is not inexpensive.